A circuit pattern is formed on a surface of a semiconductor wafer of, for example, silicon or gallium arsenide by the etching or lift off method. The wafer having patterned surface generally has its back polished by, for example, a grinder while having the patterned surface protected by a pressure sensitive adhesive sheet applied thereto. The objective of polishing the back of the patterned wafer is first to remove any oxide layer which may be formed on the wafer back in the etching conducted during the patterning and second to regulate the thickness of the patterned wafer.
The polishing of the back of the wafer having patterned surface is conducted while washing the wafer back with purified water in order to remove any generated polishing debris and in order to remove heat generated during the polishing. In the polishing of the wafer back, a pressure sensitive adhesive sheet (surface protective sheet) is stuck to the wafer surface for the purpose of protecting the pattern formed on the wafer surface from the polishing debris. After the completion of the polishing of the wafer back, the pressure sensitive adhesive sheet is stripped from the wafer surface. At the time of the stripping, it has been inevitable for part of the pressure sensitive adhesive to remain on the patterned wafer surface. Thus, it has been necessary to wash away the remaining pressure sensitive adhesive from the wafer surface. Although washing with the use of an organic solvent has been carried out for removing pressure sensitive adhesive which remains adhering to the wafer surface, it is now the mainstream practice to conduct the washing with the use of purified water in view of the consideration of environmental protection and other problems. Accordingly, the pressure sensitive adhesive which can be washed away by water is increasingly used in the above surface protective sheet.
Examples of pressure sensitive adhesive compositions for use in the above pressure sensitive adhesive sheet include those based on nonionic surfactants described in Japanese Patent Laid-open Publication Nos. 62(1987)-101678 and 63(1988)-153814 and water swellable pressure sensitive adhesive compositions described in Japanese Patent Publication No. 5(1993)-77284.
In the use of the former pressure sensitive adhesive compositions, use is made of nonionic surfactants whose molecular weight is relatively low, so that they would become the cause of residue on the wafer surface after the stripping of the pressure sensitive adhesive sheet thereby inviting the possibility of adversely affecting the circuit surface of the wafer. Such a low molecular weight component would not always be entirely removed even if the washing away operation is conducted. The surfactants employed in the above use are not those intended for antistatic performance, so that a peel electrification occurs at the time of stripping of the pressure sensitive adhesive sheet after completion of the polishing. Thus, the antistatic characteristics of the pressure sensitive adhesive compositions based on nonionic surfactants are not satisfactory.
On the other hand, the latter pressure sensitive adhesive compositions do not use surfactants and are water swellable, which include, for example, a blend of a water soluble polymer and a (meth)acrylic polymer. The molecular weights of the water soluble polymers generally used in these compositions are smaller than those of the (meth)acrylic polymers. Thus, an adhesive residue would occur on the wafer surface after the stripping of the pressure sensitive adhesive sheet. Further, the washing away of (meth)acrylic polymers is not easy although the water soluble polymers can easily be removed by washing with water.
It is an object of the present invention to provide a pressure sensitive adhesive composition which is suitable for use in a wafer surface protective sheet employed for protecting a circuit pattern formed on a wafer surface from cutting debris, etc. at the time of polishing the back of the wafer.
It is another object of the present invention to provide a pressure sensitive adhesive composition suitable for use in a wafer surface protective sheet, which exhibits an extremely low peel electrification voltage at the time of stripping of the pressure sensitive adhesive sheet and which, even if it remains on the wafer surface, can easily be removed by washing with water.